1. Field of the Invention
The present invention relates to a fuel cell system including a fuel cell stack formed by stacking electrolyte electrode assemblies and separators. Each of the electrolyte electrode assemblies includes an anode, a cathode, and an electrolyte interposed between the anode and the cathode. Further, the present invention relates to a method of starting operation of such a fuel cell system.
2. Description of the Related Art
Typically, a solid oxide fuel cell (SOFC) employs an electrolyte of ion-conductive solid oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly (MEA). The electrolyte electrode assembly is interposed between separators (bipolar plates). In use, generally, a predetermined number of the MEAs and the separators are stacked together to form a fuel cell stack.
In the solid oxide fuel cell of this type, since the operating temperature is high, at the temperature of several hundred degrees C. (e.g., around 700° C.), it is necessary to rapidly raise the temperature of the fuel cell at the time of starting operation of the fuel cell. For this purpose, Japanese Laid-Open Patent Publication No. 2004-335161 discloses a technique of heating a solid oxide fuel cell. According to the disclosure, fuel oxidizing catalyst is provided in an oxygen-containing gas channel of separators. At the time of pre-heating the fuel cell before starting operation of the fuel cell, heating means is used to heat power generation cells from the outside. A mixed gas of an oxygen-containing gas and a fuel gas of less than the lower explosive limit is supplied to the oxygen-containing gas channel, and catalyst combustion of the mixed gas is induced by the fuel oxidizing catalyst. By the heat of the catalyst combustion, the power generation cells are heated internally.
Further, in a technique of heating a solid oxide fuel cell disclosed in Japanese Laid-Open Patent Publication No. 2001-155754, a combustor is provided adjacent to an air supply channel, and another combustor is provided adjacent to a fuel gas channel. At the time of starting operation of the fuel cell, by combustion in both of the combustors, the combustion exhaust gas is supplied to the air supply channel and the fuel gas channel for heating the cell area.
Further, Japanese Laid-Open Patent Publication No. 2003-132933 discloses a fuel cell stack including fuel cells in which combustion heater plates are provided between cell plates of the fuel cell stack.
In Japanese Laid-Open Patent Publication No. 2004-335161, the fuel oxidizing catalyst is provided in the oxygen-containing gas channel. By combustion of the mixed gas in the oxygen-containing gas channel, the power generation cells are heated. In the structure, the temperature on the fuel gas channel side is considerably low in comparison with the temperature on the oxygen-containing gas channel side. Thus, because of the presence of such a temperature inhomogeneous distribution in the power generation cell, uniform heating cannot be performed, and the temperature cannot be raised rapidly.
Further, in Japanese Laid-Open Patent Publication No. 2001-155754, the combustors are provided on both sides of the fuel cell, and the temperature of the fuel cell is raised by the combustion gas from the combustors. Therefore, the fuel cell tends to be degraded easily by the combustion exhaust gas. Further, since the two combustors are provided, the overall size of the fuel cell becomes considerably large.
Further, in Japanese Laid-Open Patent Publication No. 2003-132933, the combustion heater plates are provided between the cell plates of the fuel cell stack. Therefore, the number of combustion heater plates becomes larger as the increase in the number of the cells stacked to form the fuel cell stack. Consequently, the overall size of the fuel cell stack in the stacking direction becomes considerably large, and the structure of the fuel cell stack is complicated.